1. Field of the Invention
This invention relates to magnetoresistive (MR) heads for use in the playback of magnetically recorded signals. More particularly, the invention relates to MR heads of the type, often referred to as UMR heads, wherein an elongated thin film is brought into direct contact with a recording medium and, in response to signals recorded in the medium, signal current flowing the length of the film is caused to vary.
The invention, as well as the prior art, will be described with reference to the figures, of which
FIG. 1 is a diagram illustrating a prior art practice utilizing an MR playback head,
FIGS. 2a-2c are diagrams useful in describing apparatus according to the invention,
FIG. 3 is a diagram useful in teaching a problem addressed by means of the invention,
FIG. 4 is a diagram illustrating the structure of a magnetic head according to the invention, and
FIG. 5 is a diagram illustrating the efficacy of the invention.
2. Description Relative to the Prior Art
In the interest of describing the state of the art, as well as the PROBLEM addressed by the invention, reference should first be had to the UMR head of FIG. 1: A sense current i is passed through an MR sense film 10 via contacts 12, 14. In response to signal information magnetically recorded in a recording track 16, the resistance of the film 10 varies to cause the voltage drop across the film to vary accordingly. As is fairly conventional, source of hard (vertical) axis bias field, provided for through a variety of means, biases the ordinarily longitudinal moment to an orientation, as depicted.
FIGS. 2a-2c depict schematically various typical domains which may be associated with the film 10 of FIG. 1. In response to a recorded signal field M.sub.s, the domain walls will move, expand, and contract; and, as is known, such domain variations are manifested as signal noise, viz. Barkhausen noise.
The elimination of Barkhausen noise problem may be achieved, for example, by significantly extending the length of an MR film, whereby only a wall-free single domain may intrinsically coact with a track. This can be a valid solution to the Barkhausen noise problem, but it is restricted, realistically, to head configurations wherein only a single record track is read at a given time, or a given track configuration with a significantly wide track width (.gtoreq.200.mu.m).
U.S. Pat. No. 4,547,824 discusses another technique for ridding an MR sense film of a plurality of magnetic domains, viz. by applying an easy axis bias field along the longitudinal axis of the film, whereby domain walls are swept from the active area of the film. Thus, with but a single domain existing in the film, Barkhausen domain movement is intrinsically impossible. Such an easy axis bias field is complementary to the aforenoted "moment-biasing" hard axis field usually applied in the plane of the film perpendicular to the longitudinal axis of the film.
The success of an easy axis "longitudinal" field in removing domains and accompanying Barkhausen noise in an MR sense film is due to the cancellation, by the easy axis field, of the demagnetization field associated with the MR sense film. If the MR sense film were, however, truly in a single domain state, its demagnetization field would, intrinsically, be highly non-uniform, being much larger at its ends than at its center. Thus, were a uniform easy axis field, as practiced in the prior art, to be employed to quash Barkhausen noise, it goes without saying that true cancellation of the demagnetization fields throughout the sense film would not, as a practical matter, be possible: at best one could only achieve demagnetization field cancellation over a limited central location of the film, similar to that shown in FIG. 3. (Note: To cancel demagnetization fields near the ends requires a uniform easy axis field which is much larger than necessary in the middle of the film due to non-uniformity in the demagnetization field. This can significantly reduce sensor sensitivity to signal fields.) As indicated, the sense film of FIG. 3, as magnetized, consists of a central uniformly magnetized region 20, with complex domain closure structures at the ends 22, 24. Provided these structures remain at their respective ends--and especially if they remain under their respective current contacts (12, 14)--they produce no spurious noise in the information signal associated with the MR sense film 10. There is a danger, however, that the closure structures are not always sable, and may propogate into the active central region 20 of the film 10 under the action of the applied signal field associated with the track 16. Indeed, the problem becomes progressively more severe for MR sense films which correspond to narrow magnetic track widths, typically those less than 100 microns in width.